MONDAY, May 10 (HealthDay News) -- People who sleep less than six hours a night are 12% more likely to die prematurely than those who get the recommended six to eight hours of slumber, a new study has found.

The team of British and Italian researchers also found that sleeping too much -- more than nine hours a night -- doesn't increase the risk for death but might be an important sign of a serious or potentially fatal illness.

The researchers reviewed 16 studies that included more than 1.3 million people who were followed for up to 25 years. In that time, more than 100,000 deaths were recorded among the participants, who were from Asia, Europe and the United States.

The findings provide unequivocal evidence of the direct link between insufficient sleep and increased risk of premature death, said the authors of the study, which is published in the May issue of Sleep.

"Modern society has seen a gradual reduction in the average amount of sleep people take, and this pattern is more common amongst full-time workers, suggesting that it may be due to societal pressures for longer working hours and more shift-work," Francesco Cappuccio, leader of the Sleep, Health and Society Program at the University of Warwick in the United Kingdom, said in a university news release. "On the other hand, the deterioration of our health status is often accompanied by an extension of our sleeping time."

"Consistently sleeping six to eight hours per night may be optimal for health," he added. "The duration of sleep should be regarded as an additional behavioral risk factor, or risk marker, influenced by the environment and possibly amenable to change through both education and counseling as well as through measures of public health aimed at favorable modifications of the physical and working environments."